Desorption Electrospray Ionization Mass Spectrometry (DESI-MS) is an ambient ionization technique that allows chemical analytes to be sampled from surfaces without special sample preparation. DESI-MS has been used for high throughput analysis of analytes on substrates, imaging, and online liquid sampling. FIG. 1 shows a conventional DESI-MS system. In the figure, solvent 2 is electrosprayed from the tip of capillary 4 of an electrosonic spray ionization source (not shown) and directed towards surface 14, forming charged solvent droplets 20 that are accelerated with the aid of nebulizer gas 10 that is passed through an outer capillary 8 of the electrosonic spray ionization source. Charged solvent (primary) droplets 20 released from capillary 4 form a liquid film 16 on surface 14 that eject secondary fluid droplets 22 containing analyte 18 desorbed from surface 14. Analyte 18 on surface 14 is removed from surface 14 by heterogeneous charge-transfer or droplet pick-up. Secondary charged solvent droplets 22 containing analyte ions 18 ejected from the surface are subsequently introduced at atmospheric pressure through inlet 24 of mass spectrometer 26 for analysis. Secondary droplets 22 result primarily from momentum transfer as primary solvent droplets 20 accelerated from capillary 4 by nebulizer gas 10 contact liquid film 16 on surface 14. As a result, secondary droplets 22 containing analyte ions 18 sampled from surface 14 are “splashed” towards inlet 24 of mass spectrometer 26 and surrounding areas. Splashing of droplets 22 containing analyte ions 18 is caused by collision between primary solvent droplets 20 and neutral gas 10 molecules in incoming gas jet stream 12 with liquid film 16 on surface 14, which results in transport of analyte 18 from surface 14. The splashing effect is undesirable in many applications, including, e.g., chemical imaging, because it can result in decreased detection efficiency, reduced detection limits, material transport on the surface and material loss, e.g., if charged solvent droplets containing analyte first encounter a counter electrode that is not the inlet of the mass spectrometer. Accordingly, new desorption devices and process are needed that minimize “splashing” effects at the surface, thereby maximizing analyte collection efficiency from the surface for suitable imaging and analysis of complex analytes.
The present invention provides a new apparatus and process for meeting these needs. Additional advantages and novel features of the present invention will be set forth as follows and will be readily apparent from the descriptions and demonstrations set forth herein. Accordingly, the following descriptions of the present invention should be seen as illustrative of the invention and not as limiting in any way.